1. Technical Field
The present disclosure relates to fasteners in general, and to fasteners that are adapted to accommodate environmental conditions (e.g. thermal, electrical, chemical, etc.) in particular.
2. Background Information
Fasteners are a known mechanism for attaching components together. In many instances, fasteners (bolts, screws, etc.) are made from a single material and the characteristics of the fastener material dictate the performance and durability of the fastener in a given application. In some applications, a portion of a fastener (e.g., a bolt head) may be exposed to a first set of environmental conditions and the opposite end of the fastener may be exposed to a second set of environmental conditions; e.g., a bolt head may be exposed to a high temperature environment and the threaded portion of the bolt may be exposed to a lower temperature environment. For example, gas turbine engines include an axially extending internal gas path that extends from an ambient air inlet to an exhaust where air and combustion products exit the engine. At particular points in the gas path (e.g. at or downstream of the point where combustion occurs), gases traveling within the gas path are often at a very high temperature; i.e., temperatures where the material properties (e.g., yield stress, etc.) of metallic liners and/or fasteners may be compromised, or the material subject to high temperature oxidation. In these instances, the durability and/or strength of the metallic component may be limited. To remedy the situation, it is known to use liner panels made of ceramic or ceramic matrix composite materials, which materials have substantially greater thermal capabilities. The ceramic liner panels attach to an outer panel and an annulus is formed between the ceramic liner panels and the outer panel. Typically, cooling air is directed through the annulus to cool the back side of the ceramic liner panels and the fasteners that hold the ceramic liner panels to the outer panel. When ceramic panels rather than metal panels with film cooling holes are employed as a flow path liner, a significant reduction in cooling air is required. This enables improved engine efficiency. Hence, a fastener that extends through a ceramic liner panel and an outer panel may have a first portion exposed to high temperature core gas path gases and another portion exposed to relatively cooler cooling gas.
In other applications, a fastener made from a single material may provide an undesirable energy path; e.g., most metallic fasteners are conductive of electrical energy and/or thermal energy. Using a fastener made solely from a non-conductive material (e.g., polymer or ceramic) may eliminate the undesirable energy path, but may raise different issues (e.g. susceptibility to fracture, durability, strength, etc.). Furthermore, a fastener made from a single material may have a limited useful life in certain chemical environments (e.g. caustic, corrosive, etc.), and materials that are adapted to operate in such environments may not possess sufficient mechanical properties (e.g. yield strength, etc.) to function as a fastener.